The present invention relates generally to a passive optical network (PON), and more particularly to a passive optical network and a method thereof that support a type of virtual Ethernet LAN (VLAN) services.
Today, a passive optical network based on passivity and optical characteristics, which provides high-speed communication capability at a low-cost, is an ideal high-speed, broadband access technology for the next generation.
In general, a passive optical network system is composed primarily of the following three parts: optical line terminal (OLT), optical distribution network (ODN), and optical network unit (ONU).
FIG. 1 is a diagram showing the concept of a conventional PON system. As shown in FIG. 1, the PON system has a structure in which one point is divided into multiple points. An OLT connected to the core network is installed in the station side. One OLT is connected to multiple ONUs via one or more ODN apparatuses such as optical splitters. The ONU, located in the client's side, is usually installed in the subscriber's home or office and, as necessary, one ONU can be shared by multiple users. In the PON shown in FIG. 1, a downstream frame is transmitted from the OLT to each ONU in the broadcast mode and an upstream frame is transmitted from the ONU to the OLT in the Optical Time Division Multiplexing mode. The PON system having such a network structure can provide the user with various services ranging from the conventional telephones and the Internet services to the IPTV which has recently evolved.
When broadcast frames are sent in the conventional Ethernet network where there are many terminal apparatuses in the network, the broadcast signals use up the bandwidth (broadcast stream), sometimes causing the network function to go down. The concept of a virtual LAN (VLAN) has been widely used to allow a network to satisfy the need of both QoS and network management.
A VLAN, which is created by dividing a large network into multiple logically independent subnets, does not depend on the physical configuration. Each VLAN is though of as one logical sub broadcast area, and the terminals in the area have the same VLAN identifiers (VIDs). Therefore, a broadcast frame sent from a VLAN can be accepted only by a terminal in the same VLAN. Dividing a network into sub broadcast areas, independent to each other, allows a VLAN to conveniently manage the network and, at the same time, to efficiently suppress broadcast streams that may occur in the broadcast mode.
It is very important to provide a PON system with the complete VLAN function because VLANs provide the flexible and safe network management capability.
Conventionally, a PON system that can partially support the VLAN service is already provided. For example, US patent 2005/0083950 titled “Shared LAN emulation method and apparatus having VLAN recognition and LLID management functions on EPON” discloses a PON system that can identify one type of VID. According to the idea disclosed by the invention described above, an OLT assigns a unique logic link ID (LLID), which identifies each ONU or ONU group, to each ONU. When a broadcast frame including a VID is received, the OLT sends the frame to the ONUs having the same VID.
In such a PON system which supports VLANs, communication can made only among the terminals belonging to the same VLAN and, therefore, this system has the two problems given below.
One problem is that a conventional OLT does not support communication among different VLANs. For communication among terminals belonging to different VLANs, the apparatus that manages a VLAN must translate the VID of a data frame from the communication source VLAN to the VID of the communication destination VLAN to allow a terminal in the VLAN described above to receive the data frame. However, the conventional OLT has only the function to identify a VID but does not have the function to automatically translate a VID among VLANs.
In addition, the conventional OLT does not support a service VID that is an extended VID. Although a VID, a 12-bit field, can provide the maximum of 4094 VLANs, a service provider may assign a VID to each subscriber in which case 4094 VIDs are used up immediately. According to the definition of 802.1 Q-in-Q (VLAN encapsulation) presented to IEEE802.1ad, a service provider can distribute one service VID to multiple different VLANs (that is, those VLANs belong to a VLAN identified by one service VID), and the distributed service VID is transmitted in conjunction with the specified fields added to the frame and the VID of VLAN. FIG. 8 shows the fields of the service VID. As shown in FIG. 8, another type field is added between the source MAC address field and the VID type field. The added type field has the same format as that of the VID type field except that the VID field is changed to the service VID field. The use of the service VID allows a service provider to provide 4094*4094 VLANs and thus extend the network service by encapsulated VLANs. However, the conventional OLT can neither add a service VID corresponding to the data frame when an upstream frame is received from an ONU nor identify a service VID in a downstream frame. That is, the conventional OLT cannot support VLAN encapsulation.
To solve both problems with the OLT described above, the system according to the conventional technology has a function corresponding to a switch between the OLT and the core network to allow the switch to achieve the translation function that translates data at communication time between different VLANs as well as the VLAN encapsulation function that adds and identifies a service VID. However, the switch having such functions, which is usually set up manually, requires a complex and inefficient maintenance procedure and thus significantly affects network flexibility.
Therefore, there is a need for a passive optical network (PON) system and method for fully supporting the VLAN service.